1. Field of the Invention
This invention relates to beamforming in the downlink in a cellular radio environment. In particular, this invention relates to adaptive beamforming using an antenna array at a base station and preferably single antennas at the respective user equipment (UE), with the object of improving cell throughput.
2. Description of the Related Art
Antenna array processing techniques are a key enabler for achieving the cell throughput and availability of high data rates which are likely to be required for future broadband wireless systems. Improvements in spectral efficiency may be achieved by exploiting the spatial dimension. To this end, uplink beamforming techniques are known, for example, from T. S. Rappaport, “Smart Antennas, Selected Readings” Piscataway, N.J.: IEEE, 1998 and J. E. Hudson, “Adaptive Array Principles”, IEE, 1980.
However, hitherto, much less work has been carried out on the more challenging, downlink beamforming case.
Beamforming techniques can be categorised as fixed/switched beam or adaptive beam. In the fixed/switched beam approach, a set of narrow beams is pre-defined. For each user, the most appropriate beam is chosen and the signal for that user is transmitted on the chosen beam. In the adaptive beam approach, the beam shape is not chosen from a set of fixed alternatives but is instead calculated specifically for each user. Although more complex to implement, fully adaptive beams provide greater opportunity for performance gains than fixed/switched implementations because they more accurately track the dynamics of the distribution of users within the cell.
Both the fixed/switched beam and adaptive beam categories can be further subdivided into single user or space division multiple access (SDMA) techniques. In the first case, the bandwidth assigned to each user cannot be re-used within the same sector. A beam is formed within the cell or sector of interest which concentrates the radiated energy towards the wanted user and hence minimises the interference to neighbouring cells. In the case of SDMA, the same bandwidth can be re-used within the sector providing the users are spatially differentiated. This gives rise to parallelism and hence is very effective in multiplying the spectral efficiency of the sector.